Expand version field during image_build using version_generator
    
    We can specify !VERSION_FROM_VERSION in version field during
    image_build to expand it to correct release number without any label
    information.
    
    Also implemented !RELEASE_FROM_DATE_RESPIN to provide correct
    release number. This helps to keep Atomic Host media files name
    produced by image_build during bodhi updates compose run
    consistent with nightly run.
    
    Fixes: https://pagure.io/pungi/issue/987
    Merges: https://pagure.io/pungi/pull-request/995
    
    Signed-off-by: Sinny Kumari <sinny@redhat.com>